Hypersonic vehicles are vehicles, such as aircraft, missiles, space planes, airplanes, drones, etc., capable of traveling at hypersonic speeds. As used herein, hypersonic is considered speeds above Mach 5, supersonic is considered speeds above Mach 1, and subsonic is considered speeds below Mach 1.
Hypersonic vehicles may use some type of air-breathing hypersonic engine, such as a scramjet engine, as the means of propulsion. A scramjet engine is an air-breathing jet engine in which combustion takes place in supersonic airflow. Scramjet engines rely on high vehicle speed to forcefully contract the incoming air before combustion and keep the air at supersonic speeds throughout the engine. Keeping the air at supersonic speeds through the engine allows a scramjet to operate efficiently at extremely high speeds.
A typical scramjet engine is composed of three basic components: a converging inlet, where incoming air is contracted; a combustor, where fuel is burned with atmospheric oxygen to produce heat and increase resulting combustion product pressure; and a diverging nozzle, where the hot exhaust gas is accelerated to produce thrust. Unlike a typical jet engine, such as a turbojet or turbofan engine, a scramjet engine does not use rotating, fan-like components to contract the air. Instead, the speed of the vehicle causes the air to contract within the inlet. Thus, one benefit over conventional combustion engines is there are no compressor blades or moving parts. However, since a scramjet engine lacks a mechanical compressor, these hypersonic engines require the high kinetic energy of a hypersonic flow to contract the incoming air to operational conditions. Thus, a hypersonic vehicle powered by a scramjet engine must be accelerated to the required velocity (usually about Mach 4) by some other means of propulsion, such as a turbojet, rocket engine, light gas gun, rail gun, etc., to a speed where the scramjet engine can be ignited.
For a period of time during acceleration, the speed of the hypersonic vehicle will be too low for the hypersonic engine inlet to ingest all oncoming airflow. If the flow approaching the inlet at a given supersonic speed, pressure, temperature, and angle of attack, cannot all pass through the inlet, a strong shockwave system will form in front of the inlet, reducing the flow speed and spilling a fraction of the oncoming air flow around the inlet. This creates a large inlet drag. The strong shockwave also separates the airflow boundary layer at the wall, creating a violently unsteady and noisy inlet flow behavior called “inlet buzz.” An analysis of predictions regarding “inlet buzz” using computational fluid dynamics and actual test results of “inlet buzz” in a dual-stream, low-boom supersonic inlet can be found in National Aeronautics and Space Administration, NASA/TM—2012-217612, Analysis of Buzz in a Supersonic Inlet (2012). This state of inlet operation is commonly referred to as “unstart.” A detailed description and analysis of the “unstart” process can be found in Center for Turbulence Research, Annual Research Briefs 2010, pgs. 93-103, A Numerical Study of the Unstart Event in an Inlet/Isolator Model (2010). In addition, a detailed study of the influence of boundary layers on the “unstart” process can be found in 17th AIAA International Space Planes and Hypersonic Systems and Technologies Conference, AIAA 2011-2349, The Influence of Boundary Layers on Supersonic Inlet Unstart (2011). The very high acoustic and vibratory loads produced by inlet buzz can be detrimental to the vehicle structure, onboard systems, and/or vehicle stability and control, requiring possible onerous design solutions to mitigate the negative effects. The same problem can be encountered when the hypersonic vehicle decelerates, such as upon descent to target impact for a missile.
There are various means commonly employed to avoid the negative effects of inlet buzz and unstart, depending on if the inlet of the hypersonic engine is a 2D inlet or a 3D inlet. A 2D inlet engine, such as that shown in FIG. 12 and generally designated as 300, has a rectangular inlet 310, where all walls 320 are linear in x-y planes. These 2D inlet engines 300 can employ variable geometry inlets to prevent inlet unstart by reducing inlet area ratio, commonly called contraction ratio, when the vehicle is traveling at lower speeds. In contrast, a 3D inlet engine has a curved inlet, where the walls are curvilinear and can have complicated curvatures. These inlets can provide advantages in inlet compression efficiency and are compatible with combusters having circular or elliptical cross-sections, which are more structurally efficient than combustors with rectangular cross-sections.
For a 2D inlet engine, one current means to avoid inlet buzz and unstart is to mechanically close the inlet with a rotating cowl flap until ready to start the engine, at which time the inlet flap is rotated open. The rotating cowl flap can close the inlet entirely, or open it partially, preventing unstart. Another possible means to avoid inlet buzz and unstart is the use of bypass doors in the inlet that divert a portion of the oncoming airflow into separate channels that exhaust the airflow into a low-pressure region of the vehicle. This effectively increases the throat area of the inlet and reduces inlet internal contraction ratio. While high inlet internal contraction ratio is the root cause of inlet unstarts, a high inlet contraction ratio is required to achieve high engine performance at high vehicle speeds. Therefore, while using a cowl flap or internal bypass doors on a low internal contraction ratio 2D inlet can solve the problem, this design solution is mechanically complex and increases inlet weight. However, a rotating cowl flap cannot readily be incorporated on a 3D inlet because of surface curvature and the mechanical problems of bypass doors would be daunting for 3D inlet engines.
For a 3D inlet engine, which may increase overall vehicle performance, an alternative means is required to prevent the negative effects of inlet unstart and buzz during both aircraft acceleration and deceleration. One current means for 3D inlet engines is to cover the inlet with a fairing or shroud that is ejected from the vehicle just prior to starting the hypersonic engine. However, once the fairing or shroud is ejected, there is nothing to protect the hypersonic engine when the vehicle decelerates.
Therefore, there is a need for an effective means for avoiding inlet buzz and unstart in 2D and 3D engine inlets and, in particular, in 3D engine inlets that maintains the high contraction ratio of the 3D engine inlet and that can be used during acceleration and deceleration of the hypersonic vehicle.